Insulin treatment has been shown to increase phosphoprotein phosphatase activity. Furthermore, it has been shown that the heat stable protein inhibitor of phosphatase isolated from insulin treated muscle is less inhibitory than those isolated from control muscle. The nature of this difference is unknown. The most likely alteration is phosphorylation since one of the inhibitors is phosphorylatable. Alkaline hydrolysis, chemical determination of phosphate content and immunoprecipitation will be conducted to prove this point.